Fuel, cost, energy efficiency and CO2 emission performance of PCM integrated wood fiber composite phase change material at different climates

Wood fiber is a great potential supportive material for creating a new composite the phase change materials (PCM) due to its beneficial qualities, including high sorption competency, low density, enviro -friendliness, economic effectiveness, and chemical inertness. The main objective of this paper is to study the effect of using the wood fiber/eutectic mixture of stearic and capric acid on the fuel, cost, and carbon emission-saving potentials for various PCM cases. Which experiences a phase transition within the thermally pleasant temperature range of buildings, used for the building's thermal energy storing purposes and consumption cost saving. The energy performance analysis was carried out for buildings incorporated with stearic and capric acid eutectic mixture of PCM with wood fiber-based insulation material (INS) in different climate regions. The results showed that the largest energy-saving capacity belongs to PCM5. The energy saving reaches 52.7% for PCM5 for a thickness of 0.1 m. The PCM1, PCM2, PCM3, PCM4 can provide energy saving rates of 23.5%, 34.3%, 44.7% and 50.5%, respectively. INS-PCM5 can provide about 1.74-, 1.5-, and 1.33 times larger cost savings than INS in 2nd, 3rd, and 4th regions for all fuels. The payback period varies between 0.37 and 5.81 years regarding the fuel and Region. Finally, the results indicate that the proposed composite provided a promising energy-saving potential in building applications by reducing.

www.nature.com/scientificreports/ acid with delignified wood to attain form-stable CPCM having appropriate TES properties. Delignified-wood has a porous honeycomb structure and provides better adsorption characteristics than wood. Jeong et al. 46 formed thermally improved wood flooring through microencapsulated PCM. It is observed that MWAC with MPCM could be used for wood-based flooring due to its muscular bonding strength and good energy storage capacity. Barreneche et al. 47 combined organic PCM and wood. To eliminate the leakage problem of PCM, a polystyrene solution was applied to the final product. The proposed composite provided a promising energy-saving potential in a building application. Mathis et al. 48 prepared wood boards, including biobased PCM. It is observed that wood boards provide high energy-saving potential in buildings due to significant thermal energy-storing capacity with about 22 ℃ phase change temperature.
In conclusion, many papers deal with several experimental studies and analyses about synthesizing novel PMCs, their characterization, and their incorporation into building materials. The outputs of these papers also revealed advantageous influences of the utilization of PMCs, such as improving thermal comfort and heating/ cooling energy savings.
Using wood fiber (WF) as an enviro-and eco-friendly support component for investigating a new composite PCM can significantly influence TES applications in wooden structures. Therefore, a wood fiber was combined with a eutectic combination of stearic-capric acid, which experiences a phase transition within the thermally pleasant temperature range of buildings. In this study, a novel composite PCM with wood fiber proposed was considered an energy storage component of a building. Moreover, studies have yet to focus on the effect of construction and building materials containing phase change materials on fuel types and consumption, cost saving, and emission values by considering different climates.

Methodology
PCM material. The thermal achievement of a passive structure design may be improved by using PCM as thermal energy storage. PCM-impregnated insulation material was considered for evaluation. The stearic-capric acid (SA-CA) eutectic mixture impregnated Wood fiber (WF) insulation material was supposed to improve the energy performance of a building.
The melting temperature was around 29-32 ℃ in the case of capric acid. That of stearic acid was around 69-72 ℃. The purities of capric and stearic acids were higher than 98% and 97%, respectively. The ratios of CA and SA in the eutectic mixture were 83 and 17 wt.%. The eutectic mixture melts at the temperature of 24.73 ℃. The eutectic mixture's latent heat is 179.10 J/g. The mixture's solid and liquid state densities were 983 kg/m 3 and 853 kg/m 3 . The insulation material with PCM (INS-PCM) was produced by impregnating the SA-CA mixture into wood fiber (WF) for various PCM. The WF/SA-CA mixture at the selected composition ratio was stirred in acetone for 2 h for homogenization. These eliminate the inside air of the beaker and accomplish better impregnation. The attained sol-gel was maintained in a temperature-managed vacuum oven at 30 °C for 30 min. After that, the WF/SA-CA composite was subjected to heat treatment at 80 °C for 3 h to evaporate its acetone content. After the molding process of the sample with a size of 0.02 m × 0.025 m × 0.005 m, it was then subjected to a leakage test by heated at 30 °C to decide the most significant impregnation ratio of PCM ingredient. The impregnation-molding and leakage test procedure was repeated, and for different PCM, weight contents changed from 25 to 52 wt.%. The PCM's incorporation ratio into the composite was obtained with the following: where m PCM and m WF are designated as mass quantities of the WF and PCM ingredients of the composite. m PCM was decided by considering the weight loss in the sample composite after the leakage test, and this value was used to calculate the PCM's incorporation ratio via this equation. The thermal conductivity of stearic-capric acid (SA-CA) eutectic mixture/wood fiber (WF) can be obtained by: The weight loss of 25 wt.%, 35 wt.%, 45 wt.%, 50 wt.%, and 52 wt.% PCM cases were decided to be less than 0.1% in the leakage test. Consequently, the composite sample, which contains 52wt% PCM and has a weight loss of less than 1%, was identified as a leakage-proofing sample and is called INS-PCM in this work (Fig. 1). After the wood fiber 52, wt.% PCM ratio, the amount of PCM leakage significantly increased. Therefore, 25,35,45,50, and 52 wt.% were selected for the investigation.
The energy storage specification of the PCM and PCM-impregnated wood fiber-based insulation components were defined by the Differential Scanning Calorimetry (DSC) ( Table 1). Figure 1 shows the final product (INS-PCM) and the procedure schema for preparing leak-proofing WF/SA-CA. Solar energy analysis. The PCM-impregnated wood fiber-based insulation material was considered on the structure's wall, ceiling, and floor. The PCM receives solar radiation throughout the year. Therefore, it is essential to decide on incident energy. Solar energy illuminates the structure's walls and ceiling. The following equation shows monthly mean daily (MMD) irradiation on an inclined plane 50-52 . where the H b , H d , and H are direct, diffuse, and global irradiation on the horizontal plane. The upper line on the symbols indicates the MMD values. Global solar radiation is measured using a pyranometer. R b , R d , and R r are the angle factors of direct, diffuse, and reflected radiation. The angle factor in the solar radiation equation can be rewritten as 50,51 . www.nature.com/scientificreports/ Diffuse and reflected solar radiation is associated with the tilt angle of a plane. The building surface can be considered perpendicular. The angle factor of direct radiation is a function of latitudeϕ, tilt angleβ, declinationδ, and sunset hour angle ω S 50 .
(4) q s = H b cos θ sin α −1 S + H d cos 2 (0.5β) + Hρ r sin 2 (0.5β)  www.nature.com/scientificreports/ where, ω S and ω' S are the sunset hour and sunset hour angles for the titled plane for the mean mount day, respectively. These can be calculated by 50 The MMD direct irradiation on the horizontal plane is calculated with where MMD diffuse radiation is calculated by; The clearness index is designated by the rate of MMD global irradiation on the horizontal plane to that of extraterrestrial irradiation.
where the MMD extraterrestrial radiation is obtained by; where I SC is the solar constant. n is the number of days in a year. Table 2 shows the typical days of a month, declination angle, sunset angle, sunset angle for the titled surface, and angle factor of direct radiation.
Turkey's thermal insulation standards for buildings (TS 825) divide the country into four climate areas based on the climate environments 53 . Four climate regions in Turkey were considered for evaluating a building's energy efficiency. The climate is typically milder along the coast. Mediterranean climate is prevalent throughout the shores of the Mediterranean and Aegean Seas. As a result, the midsummers are warm and arid, and the winters are mildly cold and damp. The Black Sea's coastal region experiences an oceanic climate with hot, rainy summers and cold, rainy winters-the plateau in central Anatolia experiences continental weather. Winters are chilly and covered in snow, and summers are warm and arid. Eastern Anatolia experiences harsh winters that are exceedingly cold and snowy, as well as sweltering and dry summers. The four cities chosen are Ankara, Istanbul, Erzurum, and Antalya. These are in the central Anatolian plateau, in the coastal region bordering the Black Sea, in eastern Anatolia, and the coastal region bordering the Mediterranean Sea, respectively. These cities are in the 1st, 2nd, 3rd, and 4th climate regions. Figure  Thermal analysis. A building component may include convective, conductive, and radiative heat transfer mechanisms. Heat transfer occurs over walls, windows, doors, ceilings, floors, and ventilation. The heat transfer rate for the planar surface is described in the following: www.nature.com/scientificreports/ When calculating the thermal requirement of a structure, the thermal acquisition, the thermal acquisition, containing inner ones associated with solar radiation (alternating with building orientation, location, and window area), electrical devices, and household appliances should be considered, as well as the heat loss over the components of the structure. The yearly thermal necessity of a building can be achieved by considering the energy losses and gains in the system. The convective heat transfer can be attained by using the following; The thermal conductivity of a component, k m, varies according to the conditions. The characteristic length, L c, describes the scale of a physical system. The total specific heat loss is described by [53][54][55] H T and H V indicate heat losses through building components and ventilation, respectively. The ventilation loss is determined using heat capacity c, airflow rate Vʹ, and densityρ. U comp, i and A comp,i are the total heat transfer coefficient, and the area of i is the component. The building thermal insulation requirements standard indicates that the internal heat gains can be considered 5 W/m 2 at the utmost in a typical building 53 . The internal heat gains can be obtained by totaling inner and solar thermal gains as follows; 53,56 These can be decided by considering the shading factor of window r i,m , the transmissivity through the window g i,m , and the incident irradiation I i,m for the i-direction. A w , and A n is the window and building areas in use. The thermal energy necessity of a structure can be determined annually by; 53,57 PCM absorbs energy regarding its latent heat and releases the stored energy at a lower than its freezing temperature. The inside temperatures of buildings were considered above the melting temperature in the daytime while reducing to below the freezing temperature at nighttime. The PCM-impregnated wood fiber-based insulation material was supposed to cover the building's inside walls, floor, and ceiling for maximum storage energy utilization. The energy storage in the liquid phase of PCM was ignored. Thus, the buildings can store the PCM latent heat for a day at most. PCM energy storage capacity was considered the energy gain to achieve the annual thermal requirement of the structure. The yearly energy needs of PCM-impregnated insulation-integrated buildings can be decided by; www.nature.com/scientificreports/ where h fg , m PCM , and n d are the phase change enthalpy, the amount of PCM in the structure, and the number of days in a month, respectively. The yearly fuel price of a structure is obtained by considering the fuel type, the fuel's net calorific value (NCV), fuel efficiency, and the total heat needed 58 . The energy cost of the structure heating requirement is calculated by the following; where c is the unit cost of the fuel. Natural gas, LPG, coal, fuel oil, and electricity were selected. The annual fuel amount is achieved by the yearly energy requirement rate to the fuel's net calorific value. The calorific values, efficiencies, and fuel unit cost are shown in Table 3 56,58 . The Nusselt numbers related to the building components are specified in Table 4. Table 5 shows the monthly average outdoor temperatures of four climate regions.  Table 4. Heat transfer coefficients for building components.

Nusselt number Conditions and calculations
Wall outdoor for (vertical plane) Combined laminar and turbulent Roof and ceiling (Lower hot surface)  www.nature.com/scientificreports/ Calculations of carbon emissions. The quantities of CO 2 and SO 2 emissions vary depending on combustion situations and the quantity of carbon and sulfur in the fuels. The chemical reaction for the complete combustion of any fuel with air follows 59 ; where x, y, z, t, w represents the components number of the fuels. A, B, and D are stoichiometric coefficients and are calculated according to the above reaction as follows; CO 2 and SO 2 emission amounts were calculated, and NO 2 was neglected. Therefore, the emissions per 1 kg of fuel; The emission amounts of CO 2 and SO 2 for the total fuel (M fuel ), respectively. www.nature.com/scientificreports/ The molecule weight of the fuel, M is attained for each fuel by; 59 .
The amount of carbon saving was calculated regarding fuel reduction based on the structure's yearly energy storage capacity. Table 7 shows the chemical formulas of four types of fuels. The solar radiation, thermal analysis, and carbon emission calculation were written in Engineering Equation Solver (EES) program. Thermo-physical properties of air were obtained from the EES library. These were used to attain heat loss through building components.

Results and discussion
Solar energy amount on the building surface. The latitudes of the cities in different climate regions are 36.89° N (1st), 40.98° N (2st), 37.97° N (3st), and 39.91° N (4st Region). To decide the MMD values of the solar radiation, the typical day of each month was considered 50 . The buildings were deemed located in regions with mostly light-colored buildings. Figure 3 shows the MMD solar radiation in different climate regions. The MMD extraterrestrial and global irradiation on the horizontal plane is shown in Fig. 3a,b. Global solar irradiation includes direct and diffuse irradiations on the horizontal plane and is measured by a pyranometer. These values are available in the literature 62 . The extraterrestrial solar radiation values are similar in the summer months as the difference regarding the Region gets more considerable in the winter months. The global solar radiation in different regions has more clear differences. The maximum value is observed in June. Figure 3c,d illustrates the direct and diffuse irradiations. The magnitudes of the natural solar radiation in the 1st and 2nd regions are similar. The lowest value is observed in the 4th Region. By considering the direct, diffuse, and global solar radiation, the heat flux on a perpendicular surface of the building can be decided. Figure 4 indicates the various regions' MMD solar irradiation on the vertical surface. The magnitude of the solar heat flux in the summer months is observed as comparatively low since the sun rises higher in the sky, and the solar radiation strikes the ground at a higher angle compared to the winter months. However, the 1st Region has a more significant ambient temperature, the most extensive solar radiation in the 2nd Region. It is about the characteristic condition of the location. The annual average daily values were 3.96, 4.25, 3.63, and 3.36 kWh/m 2 for the 1st, 2nd, 3rd, and 4th climate regions, respectively. These values were calculated for the typical days. Thus, annual values were obtained at 1447.5, 1554.4, 1327.95, and 1230 kWh/m 2 for the 1st, 2nd, 3rd, and 4th regions. These values are far beyond the storage capacity of the PCM-impregnated wood fiber-based insulation material.
Effect of the PCM on energy saving. The solar energy analysis indicates that the energy-storing capability of PCM can be utilized at maximum capacity. The analytical calculation was carried out to evaluate the impact of the novel PCM materials on the building energy performance. PCM-impregnated wood fiber-based insulation material (PCM-INS) was considered for thermal insulation and energy storage. Five different PCMimpregnated insulation materials were applied to the building. The material was varied regarding the thermal storage capacity. The energy storage capacities of PCM-INS panels (0.05 m × 1 m 2 × 1 m 2 ) were calculated to be 60.1 kJ, 84.9 kJ, 108.1 kJ, 120.6 kJ, and 125.3 kJ for PCM1, PCM2, PCM3, PCM4, and PCM5. The sample building is a two-story building. The PCM-INS panels were considered for the walls, ceiling, and floors. The analysis was carried out using the total capacity of the PCM energy storage since solar radiation can provide sufficient energy for the selected location. Four different locations in different climate regions were chosen for evaluation. Figure 5 shows the annual energy saving per unit area of the structure as a function of the PCM-INS thickness.
The thicknesses of PCM-INS varied between 0 and 0.1 m. The amount of energy storage is directly related to the amount of PCM in the wood fiber insulation material. The largest energy-saving capacity belongs to PCM5. It can provide an annual energy saving of 21 kWh/m 2 . It is followed by PCM4 (20.28 kWh/m 2 ), PCM3 (18.18 kWh/ m 2 ), PCM2 (14.28 kWh/m 2 ), and PCM1 (10.1 kWh/m 2 ), respectively. The great content of the PCM provides outstanding thermal absorption. However, there is another issue that ought to be dealt with. The wood fiber has www.nature.com/scientificreports/ a thermal conductivity of 0.038 W/m 2 . The eutectic mixture SA-CA PCM has a thermal conductivity of 0.14 W/ m 2 . PCM provides an advantage with the thermal energy storage characteristic, while PCM impregnation in the wood fiber reduces the thermal resistance of the insulation with growing thermal conductivity. It has an adverse effect the heat transfer. Therefore, it is required a comprehensive energy performance analysis. Thus, we can decide whether the PCM has an advantage on the building energy performance or is a thrashing. " The structure's total heat necessity was calculated regarding the thermal gains and losses of the building. The sample building was considered in four climate regions. The heat requirements of buildings in different regions drastically change regarding their environmental and climatic characteristics. Fourier's law of conduction and Newton's law of cooling calculated the conductive and convective heat losses through the wall, windows, ceiling, doors, and floor. Thermal loss through ventilation was also considered to decide the heat needed. Apart from   www.nature.com/scientificreports/ the thermal storage of PCM, solar energy and interior heat gains were considered in the evaluation. The internal heat gains arise from the heat dissipation of the electrical devices and people in the building. Figure 6 illustrates the annual heat requirements of six different buildings in four other climate regions. The INS case indicates a building using only insulation without PCM. The heat requirement range in the y-axis in the figures was set with the same to see the reduction in the heat requirements. Insulated buildings without PCM show different trends from the other buildings with PCM. The increment in the thickness reduces the heat requirement. However, the reduction in the heat requirement gradually diminishes and becomes even. Therefore, as seen in the figure, the increment of the insulation thickness (without PCM) becomes unnecessary after a certain thickness. In the 1st climate region, the difference in the curve trends becomes apparent with the increment in the thickness.  1st, 2nd, 3rd, and 4th regions, respectively. These results are exciting. The most considerable reduction in the heat requirement occurs in the fourth Region. However, PCM can provide the most extensive advancement to INS in the 2nd climate region. 3rd, 1st, and 4th regions follow it. This interesting order arises from the fact that the PCM's energy-storing capability is larger than the energy requirement of the building in the 1st Region. In the 1st Region, it is no need to enlarge the thickness after a certain value. The most attractive case occurs in the first Region due to the zero-energy building potential. As seen in Fig. 7a, the heat requirement drops below zero after a certain thickness. Namely, using the PCM, the heat requirement disappears in the first Region. It is about the low heat needed due to the warm climate characteristic of the Region. It is better to evaluate the annual saving in the Region. The annual saving rate was the ratio of energy saved to the heat required. Figure 7 44.7% and 50.5%, respectively. The effect of the PCM appears in the thickness of 0.033 m in the 3rd Region. The saving rates reach 17%, 25.4%, 33.4%, 38% and 39.8% for PCM1, PCM2, PCM3, PCM4 and PCM5, respectively. In the 4th, the saving rate is lower compared to other regions. The effect of PCM becomes apparent for the 0.044 m INS-PCM thickness. The energy saving rates are 9.8%, 15.2%, 20.5%, 23.6%, and 24.7% for 0.1 m thickness in the cases of PCM1, PCM2, PCM3, PCM4, and PCM5, respectively. These comparatively low energy-saving rates are due to harsh winter conditions' high thermal energy requirement. Even these comparatively lower rates are crucial and become significant for cumulative application. Although the lowest thermal resistance is seen in the case of PCM5, it still can provide the largest energy saving. The advantage of energy storage suppresses the adverse effect of the larger thermal conductivity of PCM5 in the 1st regions for all thicknesses and after a certain thickness in the 2nd, 3rd, and 4th regions. These thicknesses are seen in these figures. It may be better to concentrate on one PCM for the evaluation, and it is beyond argument that PCM5 shows the best performance to the other INS-PCMs.
Effect of the PCM on carbon emission. The building's carbon emissions change depending on the fuel types. It is about the carbon content in the fuel. The complete combustion of the fuels with air was mentioned in the previous section. The amount of carbon emission was calculated to be about 2.7 kg-CO 2 /kg-fuel, 3.2 kg-CO 2 / kg-fuel, 2.64 kg-CO 2 /kg-fuel, and 3.35 kg-CO 2 /kg-fuel for the coal, fuel-oil, natural gas, and LPG, respectively.
The carbon emission of these fuels can be calculated utilizing the combustion reaction. Electricity generation also requires a thermodynamic process. The amount of carbon emission for electricity generation is directly related to the generation methods. Renewable or nuclear energy may diminish carbon emissions without considering the installment or production process of the power plants.
However, natural gas, coal, or fuel-oil-based power plants have an essential share in developing and developed countries. Therefore, assuming electricity as clean energy directly relates to the generation method. In Türkiye, about 37.2% of the electricity generation is obtained by coal-fired power plants. Coal has the largest share. It is followed by NG-fired thermal power plants, with 18.6% in fossil-fuel-based plants. A small percentage belongs to fuel oil with 0.24%. The rest of the generation is hydropower at 29.2%, wind energy at 7.15%, solar energy at 3.16%, geothermal energy at 2.94 and biofuels at 1.49% 63 . Türkiye and Egypt are developing countries with significant investments in renewable energy technology in the last decades 64 .
Therefore, the electricity generation share may change over the years. In this stage, the amount of carbon released for electricity generation can be decided by considering the quantity of the fuels to produce 1 kWh of electricity, the power generation rate for 1 kWh of electricity generation, and the amount of carbon in the fuel used to generate electricity. In the last case, the amount of carbon emission for electricity generation was calculated to be 0.434 kg-CO 2 /kWh. These amounts of carbon emission may be deceiving without considering the low heat values of the fuels. INS and INS + PCM5 cases were considered to investigate the quantity of carbon emissions. Figure 8 illustrates the carbon emission of these two cases for different climate regions and various fuels. As seen in the figure, PCM impregnation can significantly reduce carbon emissions. The effect of the insulation on the carbon emission is mostly clear until a certain insulation thickness. After that, the increment in the insulation thickness becomes ineffective.
In the 1 st Region, the carbon emission reduction of the coal-using building becomes 12.75 kg per m 2 building usage area for the increment of insulation from 0 to 0.1 m without PCM. The PCM-impregnated insulation can provide a 26.58 kg reduction in carbon emission for 0.074 m thickness. It means all carbon emissions from heating purposes can be eliminated for this thickness. For the first Region, considering 0.074 m thickness can give a better comparison. The reduction becomes 11.6 kg for 0.074 m thickness in the case of insulation without PCM. The amount of carbon reduction varies depending on the fuels. In the case of electricity, the reduction becomes 8.5 kg and 19.5 kg-CO 2 /m 2 for INS and INS-PCM5 cases, respectively. The PCM can provide an 11 kg larger reduction. In the 2nd Region, the reduction of carbon emission enlarges. The decrease in coal becomes 24.25 kg and 42.36 kg-CO 2 /m 2 for INS and INS-PCM5 cases, respectively. The PCM provides about 18.1 kg-CO 2 / m 2 more reduction for coal. The advantages of INS-PCM5 compared to the INS become 13.3 kg, 11.1 kg, 8.7 kg, and 6.67 kg-CO 2 /m 2 for electricity, fuel-oil, LPG, and natural gas cases. In the 3rd Region, the reduction for coal becomes 31.6 kg and 47.5 kg-CO 2 /m 2 for INS and INS-PCM5 cases. The decrease in carbon emission enlarges for the 3rd Region compared to the 1st and 2nd regions. However, the advancement of PCM in CO 2 emission reduction gradually decreases for a higher region number. For the case of coal, the improvement becomes 15.9 kg-CO 2 /m 2 in the 3rd Region. It becomes 13.3 kg, 11.1 kg, 8.7 kg, and 6.67 kg-CO 2 /m 2 for electricity, fuel-oil, LPG, and natural gas cases. The most considerable reduction in carbon emission and the lowest advancement of PCM compared to the sole insulation case is observed in the 4th Region. The reductions become 40.37 kg and 53.68 kg-CO 2 /m 2 for INS and INS-PCM5 cases.
The advancement becomes 13.3 kg, 9.77 kg, 8.2 kg, 6.4 kg, and 4.9 kg-CO 2 /m 2 , respectively. The most significant improvement occurs in the case of coal, electricity, fuel oil, LPG, and natural gas, respectively. It is related to the amount of carbon emission. Coal causes the most significant carbon emission. The carbon emission of electricity depends on the source of the generation method. In conclusion, the PCM becomes the most efficacious  www.nature.com/scientificreports/ of the most important shareholders of the energy-saving strategy for sustainability. The cost-saving rate changes regarding the price of the energy source. This study considered coal, electricity, fuel oil, LPG, and natural gas, and their current prices were used to decide the cost saving. The price averages of the fuels were 0.25 $/Nm 3 , 0.13 $/kg, 0.096 $/kWh, 0.46 $/kg, 0.95 $/kg for natural gas, coal, electricity, fuel oil, and LPG, respectively, in Türkiye and Egypt. Figure 9 shows various regions' annual heating costs per unit area. INS and INS-PCM5 using buildings were considered for the evaluation. The figure illustrates the annual price of the heating without considering the PCM price. The figures show that the PCM can provide the most apparent advantage in the 1st Region. Insulation is the most effective and significant part of the energy-saving goal. However, increments in the thickness never eliminate the energy loss but reduce heat transfer. The annual cost becomes straight after a certain increment of the insulation thickness. Namely, its effect on cost-saving performance becomes insignificant and unnecessary. However, PCM can provide a direct advantage due to its energy storage feature. A larger thickness of the INS-PCM means larger energy storage and lower energy need. The largest heating price belongs to electricity. LPG follows it. It is about the amount of energy needed and the price. Therefore, the major cost saving arises in the electricity case. year in the 4th region, respectively. In the 2nd Region, the savings become larger than in the 1st Region. On the other hand, the amount of savings is reduced in the 3rd and 4th Regions. Although a larger energy saving was achieved in the 2nd Region, using PCM in the first Region is preferable due to zero energy building potential and less INS-PCM thickness. The amount of cost savings enlarge for a building with larger usage areas. Stearic and capric acid prices could be high for a laboratory-scale application. However, the cost of the PCM can be significantly reduced for bulk purchase of the acids. Thus, the investment becomes highly conceivable. For the payback period evaluation, the bulk purchase of the acids was considered. Figure 11 shows the payback period of the phase change materials (0.1 m) for various fuels. The increment in the thickness of INS-PCM provides a more significant amount of PCM impregnation, and this larger PCM can provide more energy storage. INS-PCM5 requires a more extended payback period than the other PCM cases.  www.nature.com/scientificreports/ On the other hand, it also can provide a larger annual energy and cost savings. The shortest payback period is observed in the building using electric heating. The payback period varies from 0.37 to 1.24 years PCM1 case in the 1st Region and PCM5 case in the 4th Region, respectively, for the electricity case. After that, the building using LPG for heating can provide the second shortest payback period. It varies between 0.44 and 1.48 years for LPG for PCM1-Region1 and PCM5 Region 4. The most extended payback period is observed for the case of natural gas. It varies between 1.72 and 5.8 years for the same issues.
The coal case follows it and varies between 1.25 and 4.2 years. The longest and shortest payback period becomes 0.37-5.81 years regarding the fuel and Region.

Conclusion
Wood fiber is a significant potential supportive component for creating a new composite PCM due to its beneficial qualities, including high sorption competency, low density, enviro -friendliness, economic effectiveness, and chemical inertness. This study used a stearic-capric acid/wood fiber (SA-CA) eutectic mixture for thermal energy storage, which experiences a phase change within the building's thermally pleasant temperature range. The study examines the PCM impact on building energy performance for net-zero energy building potential. A www.nature.com/scientificreports/ heat transfer investigation was conducted to state the building energy performance by considering the heat losses through the building components and ventilation, the heat gains through solar and inner gains, and the thermal energy storage of PCM. Five different PCM-impregnated wood fiber-based insulation materials were applied to the sample buildings in four different climate regions. The advantages of INS-PCMs were investigated in typical insulated buildings. The energy, fuel, cost, and carbon emission saving capacities were decided regarding the fuels. The following results summarize the study.
1. The largest energy-saving capacity belongs to PCM5. It can provide an annual energy saving of 21 kWh/m 2 . PCM provides an advantage with the thermal energy storage characteristic, while PCM impregnation in the wood fiber reduces the thermal resistance of the insulation by increasing thermal conductivity. Therefore, it is vital to consider the energy-saving potential of PCM along with the increment in the heat transfer rate. 2. The energy analysis indicated that the PCM-impregnated insulation material provides a significant advantage compared to the conventional insulation case in the first Region. The difference in the heat requirement between the INS and INS + PCMs gets more critical with an increment in the thickness. In the 3rd, the conventional insulation (INS) performs better than the INS-PCMs integrated building until the thickness of about 0.035 m. In the 4th region, this thickness becomes 0.048 m. The advantages of PCM appear after specific thicknesses. 3. The difference in the heat requirement between the INS and INS-PCM5 becomes 23.2 kWh/m 2 , 30.4 kWh/ m 2 , 26.72 kWh/m 2 , and 22 kWh/m 2 for the 1st, 2nd, 3rd, and 4th regions, respectively. This exciting order arises from the fact that the PCM's energy storage capacity is larger than the energy requirement of the building in the 1st Region. In the 1st Region, it is no need to enlarge the thickness after a specific value. The most attractive case occurs in the first Region due to the zero-energy building potential. 4. The energy saving reaches 52.7% for PCM5 for a thickness of 0.1 m. The PCM1, PCM2, PCM3, PCM4 can provide energy saving rates of 23.5%, 34.3%, 44.7% and 50.5%, respectively. 5. The carbon emission analysis showed that the advancement of PCM in CO 2 emission reduction gradually decreased for a higher region number. The largest reduction in carbon emission occurs in the case of coal, electricity, fuel oil, LPG, and natural gas, respectively. 6. The PCM-impregnated insulation can provide a 26.58 kg reduction in carbon emission for 0.074 m thickness.
In the 2nd Region, the PCM offers about 18.1 kg-CO 2 /m 2 more reduction for coal. 7. The most significant reduction in carbon emission and the lowest advancement of PCM compared to the sole insulation case is observed in the 4th Region. In conclusion, the PCM becomes the most efficacious in the case of the 1st Region, then the 2nd, 3rd, and 4th regions, respectively. In any case, PCM can provide a significant advantage in reducing carbon emissions. 8. The cost analysis showed that, in the 1st Region, INS-PCM5 provides 0.72 $, 2.43 $, 1.3 $, 2.03 $, and 0.52 $/ m 2 year saving compared to INS for coal, electricity, fuel oil, LPG, and natural gas, respectively. In the 2nd, 3rd, and 4th regions, the advantages appear after a certain thickness; these thicknesses are 0.022 m, 0.033 m, and 0.044 m, respectively. INS-PCM5 can provide about 1.74-, 1.5-, and 1.33 times larger energy saving than INS in the 2nd, 3rd, and 4th regions for all fuels. 9. The shortest payback period is observed in the building using electric heating. INS-PCM5 requires a more extended payback period compared to the other PCM cases.
The most extended payback period is observed for the case of natural gas. It varies between 1.72 and 5.8 years for the same issues. The coal case follows it and varies between 1.25 and 4.2 years. The payback period varies between 0.37 and 5.81 years regarding the fuel and Region.

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.